The present invention relates to a sputter etch process for precleaning semiconductor wafers that have a top layer with contact holes and/or vias formed therein.
Sub 0.5 micron straight walled contacts with high aspect ratio (i.e., ratio of depth to width &gt;2) may be metallized with the use of collimated Ti/TiN PVD (Physical Vapor Deposition) technologies, followed by CVD-W (Chemical Vapor Deposition with tungsten), PVD Al/reflow or cold/hot PVD Al to completely fill the contacts. As illustrated by FIG. 1, however, a straight contact side wall 10 and a sharp upper corner 12 cause a re-entrant structure or overhang 14 in the coherent Ti/TiN deposition.
Although collimated PVD has proven to be able to cover the entire bottom of a contact with vertical sidewalls, parts of the contact walls and base may remain practically void of material if the contact sidewall is (partly) re-entrant. In mass production of devices, it may prove to be difficult to guarantee complete absence of re-entrant structures in the contact hole. For example, re-entrant walls may originate from using etches having isotropic etch components on Si, silicides or the dielectric stack. Although a thinner, or even completely absent Ti contact layer on small areas around the contact periphery is not likely to have serious impact on device performance, absence of barrier material or a discontinuity in the microstructure of the barrier in these regions can lead to catastrophic junction failures due to interaction with the WF chemistry or the Al metallization. For this reason, one typically requires the TiN deposited in the bottom regions of the contact to have a thickness of at least 200 .ANG. (20 nm) to ensure reliable protection of the contact. For typical collimated PVD processes and for 2/1 aspect ratio contacts this would correspond to a TiN thickness on the field of about 700 .ANG. (70 nm). Note that a collimated PVD layer itself will become re-entrant in the top region of the contact as its thickness becomes a significant portion of the width of the contact. This may lead to void formation during subsequent Al deposition or W plug fill.
The re-entrant structure also causes other difficulties during subsequent W deposition. For example it can cause "volcanoes" which are characterized by the peeling away of the TiN layer away from the wafer starting at the narrow region below the re-entrant structure. It also makes it more difficult to fill contacts with PVD Al/reflow or cold/hot PVD Al.
It is known that facetting or rounding of the top corners of the contact hole can prevent or greatly diminish the formation of re-entrant profiles after collimated Ti/TiN PVD deposition (e.g. see A. Ohsaki et al., "Collimated Sputtering of TiN/Ti for ULSI", Proc. 2d International Symposium on ISSP '93, Tokyo (1993)). It is also known that a low pressure Ar sputter etch, such as is frequently used to clean the bottom areas of contacts, will also facet the top corners of the contacts. The present inventors have discovered, however, that with a low pressure Ar sputter etch, it is as a practical matter impossible to create enough facetting on the top corners while still limiting the amount of material etched on the bottom of the contact. This is particularly true in production runs where high throughput is required.